Ice-maker control circuit with slab limiting control

ABSTRACT

An ice maker control circuit for use with an ice maker of the type including an inclined refrigerated plate which is energized during a slab formation cycle during which water is continually circulated across the inclined refrigerated plate and wherein ice slabs released from the plate are directed to a cutting grid for melting the slab into a plurality of cubes of ice; the control circuit including a pair of ice thickness sensing thermostats each being associated with a transistorized control switch for selectively energizing first and second relay control switches in the water circulating pump circuit of the unit; the sensing thermostats being sequentially conditioned to terminate the recirculation of water across the slab under conditions wherein a slab release cycle fails to initiate because of the presence of a previously formed slab on the ice cutting grid.

United States Patent Canter ICE-MAKER CONTROL CIRCUIT WITH SLAB LIMITING CONTROL Nov. 11, 1975 Primary E.\'mm'ncrW. E. Wayner Altar/1c), Agent 0r Firirll C Evans [57] ABSTRACT An ice maker control circuit for use with an ice maker of the type including an inclined refrigerated plate which is energized during a slab formation cycle during which water is continually circulated across the inclined refrigerated plate and wherein ice slabs released from the plate are directed to a cutting grid for melting the slab into a plurality of cubes of ice; the control circuit including a pair of ice thickness sensing thermostats each being associated with a transistorized control switch for selectively energizing first and sec- 0nd relay control switches in the water circulating pump circuit of the unit; the sensing thermostats being sequentially conditioned to terminate the recirculation of water across the slab under conditions wherein a slab release cycle fails to initiate because of the presence of a previously formed slab on the ice cutting grid.

4 Claims, 4 Drawing Figures 4 //AA// l i 2w 1v A?! Z A o 5 ii; w Y

i fh i A! 25 7/0 US. Patent Nov. 11,1975 Sheet 1 of2 3,918,267

oooo

U.S. Patent Nov. 11, 1975 Sheet 2 012 3,918,267

ICE-MAKER CONTROL CIRCUIT WITH SLAB LIMITING CONTROL This invention relates to ice maker control circuits and more particularly to circuits for selectively controlling ice slab making and ice slab releasing cycles of operation in an inclined plate ice maker wherein means are included to prevent the excessive buildup of thickness on an ice slab formed on the refrigerated plate during the ice slab making cycle of operation.

US. Ser. No. 416.68I filed Nov. 16,1973, now US. Pat. No. 3,859,813 to James A. Canter discloses an improved control circuit for selectively energizing the drive motor of a water recirculation pump for directing water across a refrigerated plate during an ice slab formation cycle of operation in an automatic ice maker. The ice slab formation cycle is terminated in response to operation of an ice thickness sensor mercury thermostat that is located above the refrigerated plate which has an ice slab built-up thereon as water circulates thereacross. When a desired ice thickness is appreached, the temperature of the sensor is reduced below a predetermined temperature for example, in the range of 35.5F. Under these condition a biasing circuit for a semiconductor switching device is conditioned to operate a relay controlled switching device that deenergizes the energization circuit for the drive motor of the water recirculation pump and concurrently completes an energization circuit for a solenoid operating a hot gas recirculation valve in the refrigerant circuit for the refrigerated plate thereby to initiate a defrost cycle of operation.

The release of the ice slab is detected by a tilt plate mercury switch mounted on an energized grid that receives the release slab for cutting it into a plurality of individual cubes by melting the slab along the points of contact of the slab with the energized grid plate. The individual cubes are then directed into a bin storage system. In the aforesaid control system the grid switch is connected in circuit relationship with a base electrode of the semiconductor switching device and is operative when the slab of ice has slipped from the inclined refrigerated plate onto the grid to turn off the semiconductor switching device to thereby condition the relay control switch into an ice making mode of operation wherein the hot gas solenoid energization circuit is open and the energization circuit for the drive motor of the recirculating water pump is closed to pro duce a subsequent ice slab forming cycle of operation.

While the aforesaid circuit is suitable for its intended purpose, under certain operating conditions a slab of ice released from an inclined refrigerated slate may become lodged in a manner to maintain the tilt type mercury switch in a closed position. This will condition the semiconductor switching device to retain the semiconductor switching device biased to remain off notwithstanding the operative condition of the ice thickness sensor mercury thermostat. Thus, in a subsequent sequence of operation. the ice thickness sensor thermostat is initially returned to a closed position by heating the refrigerated plate by the hot gas defrost phase of operation and as the recirculated water passes over the refrigerated plate. ice thickness will build up thereon to a point where the ice thickness sensor mercury thermo stat temperature will indicate an ice thickness at which an ice release operation will occur.

However, since the tilt-type grid switch is maintained closed by the blocking slab of ice the semiconductor switching device will remain reversely biased and as a result water recirculation will continue across the ice slab to produce an excessive ice slab thickness.

An object of the present invention is to provide an improved control circuit for preventing the buildup of excessive ice thickness when a tilt-type mercury switch for turning off a semiconductor switching device in the circuit is blocked closed by a partially released slab of ice by the provision of means for sensing an excessive buildup of ice on the refrigerated plate and operative to deenergize the drive motor energization circuit for a water recirculating pump independently ofthe operation of the tilt-type mercury switch for sensing the presence of a slab of ice on the ice melting grid of the ice maker.

Still another object of the present invention is to pro vide an improved solid state control device for sensing the buildup of a predetermined thickness in a slab of ice including a first ice thickness sensor mercury thermostat cooled by the predetermined ice thickness and operative to condition a first solid state switching device to operate a first plurality of relay control switches to concurrently open an energization circuit for the drive motor of a water recirculating pump while completing an energization circuit for a hot gas solenoid operated valve to direct hot gas refrigerant to an evaporator coil on the refrigerated plate to initiate a slab release phase of operation and wherein the circuit includes a trip type mercury switch for sensing the release of the slab as it moves onto a slab cutting, electrically energized grid of the ice maker and wherein a second ice thickness sensor mercury thermostat is operative to detect a buildup of ice above a desired predetermined thickness under conditions where the released slab of ice blocks the trip type mercury switch into a closed position so as to maintain the first semiconduc tor switching device biased to condition the relay oper ate switch continuously in an ice slab making position, a second solid state switching device being forwardly biased when the second ice thickness sensor detects an excessive buildup of ice on the plate to energize second relay control means for terminating recirculation of water across refrigerated plate until the switch blocking ice slab is dislodged thereby to condition the tilt type grid switch to a normally open position for resuming normal cycles of ice slab formation and subsequent defrost cycles for release of ice slabs from the refrigerated plate.

Further objects and advantages of the present invention will be apparent from the following description. reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

FIG. I is a diagrammatic view of an inclined refrigerated plate ice maker with cutting grid for dividing the slab into individual cubes including the present invention;

FIG. 2 is an enlarged, vertical cross-sectional view of an ice-thickness sensor mercury thermostat used in the present invention;

FIG. 3 is another embodiment of a sensor in vertical section;

FIG. 4 is a circuit diagram including the electrical components of a preferred embodiment of the present invention.

Referring now to the drawings, an ice maker 10 is illustrated in FIG. I of the type including an inclined refrigerator plate assembly I2 located above and to one side of a grid assembly I4 which receives an ice slab from the refrigerated plate assembly 12 following an ice slab formation cycle of operation thereon. During the slab making cycle a water recirculation system I6 is operated to recirculate water across the inclined refrigerated plate I2 to be frozen into ice that adheres to the plate I2. Water not frozen into ice during the recirculation over the refrigerated plate assembly plate 12 returns to a water reservoir 18 and is recirculated therefrom by a pump 20 driven by an electric motor 22. The water recirculation circuit is from the reservoir I8 through a pump inlet 24 thence through a pump outlet 26 connected by a water supply conduit 28 to a distributor header 30 on the inclined refrigerated plate assembly I2.

During the freeze cycle wherein an ice slab is built up on the plate assembly I2 a refrigerant system is operated to cool the plate assembly I2. More particularly the refrigeration system includes a compressor 32. The compressor 32 is connected by an outlet conduit 34 to one end of a condenser 36 for cooling hot discharge refrigerant gas from the compressor 32. The cool gas is expanded across a suitable restrictor valve 38, or equivalent capillary tube, and the refrigerant is thence passed through an evaporator coil 40 on the bottom of the inclined plate assembly I2 for reducing the temperature thereof below freezing. The return conduit 42 from the evaporator 40 is connected to an inlet of the compressor 32. The refrigerant circuit further includes a hot gas bypass conduit 44 having a solenoid valve 46 therein which is under the control of a solenoid coil to connect the outlet conduit 34 from the compressor 32 across the condenser 36 and valve 38 thereby to direct hot gas through the evaporator coil 40 during a slab release or defrost cycle of operation.

The above illustrated ice maker apparatus 10 is representative of automatic ice making apparatus of a type more particularly set forth in the U. S. Pat. No. 2,784,563 issued Mar. I2, I957, to Baker wherein detail explanation of the configuration and component makeup of such ice maker I0 is more specifically set forth. The above explanation, however, will suffice for an understanding of the control circuit of the present invention.

Referring now to FIG. 4, a control circuit 48 is illustrated which includes a 115 volt power supply indicated by lines L-I and L-2. The circuit includes a double pole, double throw service switch 50 having a first movable conduit 52 connected by a conductor 54 to line L-l. The movable contact is engageable with a first fixed contact 56 connected via conductors 58, 60 to an ice collecting bin thermostat electric switch having a movable contact 62 and a fixed contact 64. The bin thermostat switch is maintained normally closed until a predetermined level of ice is accumulated in a bin storage region such as the storage cavity 66 illustrated in FIG. I. A conductor 67 connects the fixed contact 64 of the bin switch to one side of an overload thermostat 68 that is connected by means of a conductor 70 to the electric motor windings 72, 74 of motor compressor unit 32. Leads 76, 78 connect the winding 72, 74, respectively, to a conventional start relay 80 including a coil 82 and a movable contact 84 that is operatively associated with a fixed contact 86 connected by a conductor 88 to a motor start capacitor 90 thence through a conductor 92 to a terminal 94 energized by the line L-2 of the IIS volt power source. 8

Additionally the circuit includes an energization circuit for the pump drive motor 22 which runs from the service switch contact 52 through the previously described circuit including conductor 67 to a terminal 96 that is connected by a conductor 98 to a movable switch component I00 of a relay controlled multiple switch device I02 that will be operated in accordance with the formation of ice slabs on the assembly I2 to selectively energize the pump drive motor 22 and the solenoid operated hot gas solenoid 46 to control both the ice slab buildup freeze cycle and slab release or defrost cycle of operation.

The movable contact I00 is illustrated as being operatively engaged with a fixed contact 103 connected by a conductor I04 to a second fixed contact I06 ofa normally closed relay operated switch 108 that serves as an ice thickness control switch in a manner to be discussed. The movable contact IIO of the switch 108 is in turn electrically connected by a conductor 112 to a second movable contact H4 in the service switch 50 that is connected by a fixed contact II6 to a power conductor 118 from the switch 50 to one side of the motor winding I20 of the pump motor 22 having the opposite side thereof connected by means of a conductor 122 to the power conductor 92 and line L-2.

The relay control switch 102 further includes a back contact 124 that is in contact with the movable contact I00 which is moved from the fixed contact 103 to interrupt the power circuit to the winding I20 during a slab release or defrost cycle of operation. When contact I24 is engaged by contact I00 an energization circuit for the hot gas solenoid valve 46 is completed from wire L-I through the aforementioned circuit leading to the movable contact I00 thence from the fixed contact I24 through a conductor I26 connected to one side of a coil 128 that, when energized, will open the solenoid valve 46. The opposite side of the coil I28 is connected by a conductor I30 to the power conductor 92 thence to line L-2. Thus the movable contact I00 of the relay switch 102 serves to selectively energize the pump during the freeze or slab formation cycle of operation and to open that circuit and complete a circuit through the coil I28 so as to condition the valve 46 to direct hot gas through the evaporator 40 to release a form slab therefrom during a slab release or defrost cycle of operation.

The aforesaid basic switching operation of the movable contact is under the control of a switching and sensing circuit 132 that includes a step-down transformer 134 having its primary connected by wires 136, 138 to the power lines L-I and L-2. The transformer I34 steps down the line voltage to a reduced voltage in the order of I4 volts suitable for heating the cutting wires 140 of the grid 14 such that when a slab of ice slides automatically onto the cutting grid 14 following a slab release or defrost cycle it will be cut into a plurality of ice cubes which will fall through the grid into the storage bin cavity 66. An example of this cutting operation is more specifically described in the above mentioned US. Pat. No. 2,784,563 to Baker.

The electric grid wires 140 are energized by conductors I42. 144 connected across the secondary of the transformer I34. Line I44 is shown fused at I46. A small amount of current is drawn from the secondary of the transformer to produce a low-voltage requirement for feeding the wires of the grid cutting wires I40 at a reduced voltage level that is also suitable for use in the control of the relay control switch 102 for establishing the freezing and defrost cycles of operation in the ice making sequence. The secondary of the tran former is connected to a full wave bridge rectifier 148 having diodes 150, I52, 154, 156 as well as a filter capacitor 158 to produce a direct current voltage for the control circuit 132. A full wave rectified power supply is illustrated but it will be appreciated that a half wave rectification ofthc transformer secondary could also be used in the control circuit 132 within the scope of the present invention.

The control circuit 132 includes a wire 160 that connects one terminal 162 of the bridge 148 to one side of an operating coil 164 in the switch 102. The opposite terminal of the coil 164 is connected by a conductor 166 to the collector electrode 168 of an npn transistor 170. The transistor 170 further includes its emitter electrode 172 connected by means of a conductor 174 to a terminal 176 of the bridge 148 opposite to the terminal 162.

The biasing circuit to the base of the transistor 170 is connected from the conductor 160 at ajunction 178 thereto and includes a base resistor 180 in series connection with the conductor 182 to the base of the transistor 170. A second conductor 184 connects the biasing circuit to one side of an ice grid switch 186 having the opposite side thereofconnected by means ofa conductor 188 to the terminal 176.

The ice grid switch 186, in the preferred form, is a pivotal or tilt type mercury switch having a sealed, tubular-like glass housing with liquid mercury and electrical contact therein sealed as shown in US. Pat. No. 2,887,852, issued May 25, 1959, to Thomas for an Ice Maker. As disclosed by the Thomas patent, when an ice slab slides onto the ice cutting grid 14 the switch 186 is actuated to a closed position. In the preferred embodiment the switch 186 is normally maintained open. The biasing resistor 180 of the transistor 170 is connected by a conductor 189 to the cathode of a diode rectifier 191 while a conductor 193 connects the anode of the diode to the transistor collector 168. It should be noted that the diode is not required so long as the transistor 170 has a high enough breakdown voltage. The base circuit for the transistor 170 is further defined by a conductor 190 that is electrically connected to a movable contact 192 in the relay operated switch 102. The movable contact 192 is movable with respect to a fixed contact 194 connected by a conductor 196 to a first electrode 198 of a mercury slab thickness temperature sensor switch 200 having a second terminal 202 thereon connected to a junction 203 thence to a conductor 205 to the terminal 176 of the bridge power source 148. The sensor switch 200, as best seen in FIG. 2, includes a glass bulb 204 with a mercury pool 206 therein (represented by movable switch 206 in FIG. 3) which is located in spaced relationship to the inclined refrigerated plate assembly 12 to sense the temperature of water flowing thereacross and which expands or contracts to control current flow between electrodes 198, 202.

The circuit further includes a second mercury slab thickness temperature sensor switch 212 like switch 200 having a first electrode 214 connected to the junction 203 and having a second electrode 216 thereon. The switch 212 also includes a glass bulb and a pool of mercury represented by movable switch 218 in FIG. 3

located in spaced relationship from the refrigerated plate assembly 12 a greater distance than the bulb of the sensor 200 for sensing a buildup of ice slab thickness in excess of a desired ice thickness.

An npn transistor 220 controls an energization cir cuit to the relay operated switch 108. More particu larly, the energization circuit is electrically connected from the junction 178 by conductors 222, 223 electrically connected to one side ofa solenoid coil 224 in series relationship with the collector emitter electrodes of the transistor 220. A conductor 226 electrically connects the transistor 220 to the terminal 176. hi the illustrated arrangement the biasing circuit for the transistor 220 is from a junction 228 between the conductor 222 and the coil 224 to a base resistor 230 thence through a conductor 232 electrically connected to the second terminal 216 of the mercury thermostat switch 212. The operation of the ice maker unit 10 includes an ice making mode wherein the pump motor 22 is energized. and the mercury ice thickness control thermostat sensor switches 200 and 212 are closed, and the mercury grid switch 186 is opened.

During this mode of operation. the relay control switches 102, 108 are in the positions shown in FIG. 4 ofthe drawing so as to maintain the energization circuit for the motor windings 72, 74 of the compressor 32 whereby refrigerant is circulated through the evaporator coil 40. Thus water will be recirculated across a refrigerated plate assembly 12 by the apparatus described above and a buildup of ice will occur on the assembly until the ice slab thickness reaches a temperature where the sensor switch 200 is cooled to a desired temperature which in the preferred embodiment is in the range of about 35.5F.

Under these condition, the sensor switch pool 206 contracts to open the circuit across electrodes 198, 202. This causes the emitter base junction of the tran' sistor 170 to be forward biased by the current flow in the resistor 180. This causes the transistor 170 to be switched on to complete an energization circuit for the coil 164 from the junction 178 through the collector emitter terminals of the transistor 170 back to the terminal 176. Energization of the coil 164 will trip the double pole/double throw relay operated switch 102 to cause the movable contact to move to the fixed contact 124 thereby to interrupt the energization circuit for the water pump motor winding 120. Concurrently, it completes an energization circuit through the conductor 126 to the hot gas solenoid coil 128 to initiate a defrost and ice slab release cycle of operation.

At the same time, the movable contact 192 of relay operated switch 102 is shifted from the fixed contact 194 to open the circuit through the ice slab sensor thermostatic switches 200, 212 thereby to lock this control circuit in a slab release mode of operation. While the ice slab release cycle is in operation, the hot gas solenoid valve 46 will direct hot refrigerant gas through the evaporator coil 40 thereby to warm 'the refrigerated plate assembly 12 to cause the mercury sensor thermostatic switch 200 to warm and reclose. When the ice slab is released from the refrigerated plate assembly 12 it slides onto the cutting grid 14 to cause the mercury grid switch 186 to pivot and close thereby to reverse the bias emitter circuit of the transistor so that it will not conduct. This will drop out the relay coil 164 to return the movable switches 100, 192 to the position illustrated in FIG. 4 thereby to condition the circuit for a new ice making cycle of operation.

Under certain circumstances. a slab of ice in a previous ice release operation may be positioned so as to prevent a released slab ofice from sliding offthc refrigerated plate assembly 12. Under this circumstance, with the solenoid controlled relay operated switch 102 returned to an ice making cycle of operation, water will be recirculated across the retained ice slab on the refrigerated plate to produce an excessive buildup of ice thickness. in accordance with the present invention, the ice thickness sensor mercury thermostat switch 212 will be reduced in temperature by the excessive ice buildup on the retained slab and will eventually be cooled to a temperature in the range of 35.5F to refleet the presence of an excessive buildup of ice on the refrigerated plate assembly 12. This will cause the emitter base circuit of the transistor 220 to be forwardly biased thereby to switch the transistor 220 on to complete the energization circuit for the coil 224 thus operating the relay switch 108 to an open position to deenergize the energization circuit for the water pump drive motor 22. Thus, the ice buildup will be stopped.

When the slab of ice on the grid that blocks release of the slab of ice on the refrigerated plate assembly 12 is either melted free or otherwise removed, the slab switch 186 will reopen thereby to start a new slab release cycle ofoperation to place the unit back in a normal operation sequence.

The dual temperature sensor switches 200, 212 can be replaced by a single sensor unit 234 as shown in H6. 3. It is in the form ofa single mercury column 236 having an outer glass sheath 238 around a mercury pool 240 which is spaced above plate 12 to sense slab thickness thereon. When ice slab thickness cools the pool 240 to around 35.5"F the mercury column will contract to open a circuit between electrodes 242, 244 corresponding to electrodes 198, 202 in FIG. 2. When the slab thickness exceeds a desired thickness the pool 240 is further cooled and the mercury column further contracts to open a circuit between electrodes 244, 246 corresponding to electrodes 214, 216 in FIG. 4. The sequence of circuit control corresponds to that set forth with the use of two switches 200, 212 to produce initiation of defrost, ice slab release when electrodes 242, 244 are opened, and ice build-up termination when electrodes 244, 246 are opened.

While the embodiments of the present invention, as herein disclosed, constitute a preferred form, it is to be understood that other forms might be adopted.

What is claimed is:

l. A slab thickness limiting control circuit for an au tomatic ice maker of the type having an inclined refrig erated plate cooled by a refrigerant circuit and with water circulated thereover by an electric motor driven pump to build up a finished slab of ice thereon and further including an inclined ice cutting grid to receive the ice slab from the refrigerated plate when released therefrom by hot gas defrost means and with the grid serving to cut the slab into individual cubes comprising; first and second mercury ice thickness control thermo static control switches, a normally opened grid switch being closed when a slab of ice is receiver on the cut ting grid from the freezing plate. a first transistor having a base, an emitter and a collector, a second transistor having a base, an emitter and a collector, a DC power source including first and second terminals thereon. means including first and second biasing resistors connected respectively to the base of said first transistor and the base of said second transistor. normally closed first switch means for connecting the base of said first transistor to said first mercury ice thickness control thermostatic control switch. means including second switch means having a normally closed contact for energizing the refrigerant circuit to cool the plate and a normally open contact operable to a closed position for defining an electrical path for conditioning the hot gas defrost means to defrost the refrigerated plate during a slab release cycle of operation, said first mercury ice thickness control thermostatic switch being positioned a first preselected distance above the refrigerated plate to detect a buildup ofa first predetermined thickness of ice in the slab and operative to open to produce a bias on the base of said first transistor when the grid switch is normally open to cause said transistor to be forwardly biased, means including a coil connected in circuit relationship with the emitter and collector of said first transistor and operable when said first transistor is conductive to open said normally closed first switch means and to shift said second switch means from its normally closed contact to its normally open contact, said second ice thickness control ther mostatic switch being positioned a preselected greater distance above the refrigerated plate than said first ice thickness control thermostatic switch to detect an excessive slab thickness, means including said second ice thickness control thermostatic switch for producing a negative bias on the base of said transistor to cause said second transistor to be triggered conductive, a third normally closed switch means in series with said first switch means and being maintained normally closed to complete the water pump motor circuit, means responsive to said second transistor being triggered conductive to open said third switch means to deenergize said water pump when an excessive ice buildup is detected by said second ice thickness control thermostatic switch.

2. A slab limiting control circuit for association with a solid state refrigeration control circuit of the type including a first power transistor forwardly biased by means including an ice thickness sensor mercury ther mostat to selectively position a solenoid operated relay to energize either a water circulating pump motor or a hot gas recirculation valve and wherein the base circuit of the transistor is connected to a switch that detects release of a slab of ice from an evaporator to direct a reverse bias signal to the transistor to cause the solenoid operated relay to be positioned in a water circulation mode when the slab of ice has been released from the evaporator plate the improvement comprising; circuit means including a second ice thickness sensor mercury thermostat switch for detecting an excessive buildup of ice slab thickness on the refrigerated plate, a second transistor having a base collector and emitter, circuit means including a biasing resistor in circuit relationship with said second ice thickness sensor mercury thermostat, normally closed switch means for completing the energization circuit for the water pump motor, circuit means connected across the collector emitter of said second transistor including means for operating said normally closed switch means to deenergize the water pump motor when said second transistor is turned on for current flow across its collector emitter electrodes. said second ice thickness sensor mercury thermostat being operatoc to trigger said second transistor to complete the conducting circuit between its collector and emitter to condition said normally closed snitch means to terminate cncrgization of said water pump motor thereby to prevent excessive build-up of ice slab thickness on the refrigerated plate when the slab switch is maintained closed during a refrigeration cycle of operation.

3. A control circuit for preventing the excessive buildup of ice slab thickness on a refrigerated inclined plate over which water is circulated by an electric motor driven recirculating pump during a refrigeration phase and wherein an electrical defrost means is associated with the refrigerated plate to remove a slab of ice therefrom, the released slab of ice being received by an energized electrical grid for cutting the slab into a plu rality of cubes of ice comprising: Circuit means defining an energization circuit across a power source for the pump motor including first and second switch means, first and second electrically energizable solenoid means for positioning said first and second switch means to control the energization of said pump motor circuit, second circuit means for energizing the defrosting means including a back contact selectively engageable by said first switch means when operated to deencrgize the pump motor, means for selectively controlling cnergization of said first solenoid means including a first transistor having a base, electrode and emitter electrodes connected in circuit with said first solenoid means, means for directing a biasing current to the base of said first transistor including a normally closed third switch actuated by said first solenoid means and further including a first ice thickness sensing mercury thermostat, means including a tilt switch responsive to movement of a slab of ice onto the cutting grid having one terminal thereof connected to the base of said first transistor and the opposite terminal thereof connected to the power source and operative when a slab of ice has been released onto the grid to terminate the base biasing signal to said first transistor to interrupt the energization circuit for said first solenoid, and means for preventing an excessive buildup of ice on the refrigerated slab including a second ice thickness sensor mercury thermostat positioned to detect an excessive buildup of ice slab thickness, 21 second transistor having base, collector and emitter electrodes, said second solenoid means being connected across the collector emitter electrodes of said second transistor for energization across the power source, biasing means including said second ice thickness sensor mercury thermostat for directing a signal to the base of said second transistor when an excessive ice slab thickness is de tected to condition said second transistor to energize said second solenoid means thereby to condition said second switch means to terminate energization of the water pump motor when said slab switch is maintained closed following a previous defrost cycle,

4. A control circuit for preventing excessive buildup of thickness in a slab of ice on a refrigerated plate in an automatic ice maker of a type including a refrigerated plate having water recirculated thereacross by an electric motor driven pump to form an ice slab thereon and with an ice harvest cycle being initiated when a predetermined ice slab thickness occurs and including means for defrosting the refrigerated plate to cause the formed ice slab to slide onto an ice cube cutting grid,

the control circuit comprising in combinutioril a first ice thickness control thermostat switch, a grid switch actuatablc when a slab of ice slides from the refrigerated plate onto the cutting grid, at first npn transistor having a base, an emitter, and a collector, a direct Llll'- rent power source having first and second terminals one of said terminals being connected to said grid switch and said first thermostat switch. means including a biasing resistor for connecting the other of said power source terminals to the base of said first transistor, normally closed first switch means connecting said transistor base through said first thermostat switch to the other power terminal, second switch means having a normally closed contact for energizing means for circulating water across the refrigerated plate and a nor mally open contact, means including said normally opened contact to define an energization circuit for operating the defrost means for the refrigerated plate, said biasing resistor being selected so that when a pre determined temperature is sensed by siad first ice thickness control thermostat switch a current flow will be directed through the base emitter circuit of said first transistor to cause it to be conductive between the collector-emitter circuit thereof, first solenoid means connected in circuit with the collector-emitter circuit of said first transistor and cnergizable when said first transistor is triggered conductive to condition said first switch means to open and to condition said second switch means to close the normally open contact thereof thereby to terminate water circulation and to initiate defrost and an ice slab harvest cycle, a normally open ice slab grid switch connected between the base of said first transistor and one of the power source ter minals and operative when the slab of ice has been released to reverse bias the base emitter circuit of said first transistor to terminate energization of said first solenoid means thereby to terminate defrost of the refrig erated plate and to rcinstitute recirculation of water across the refrigerated plate, and means for preventing excessive buildup of ice when the normally open slab sensing grid switch is closed by the presence of an uncut ice slab on the grid including a second ice thickness control thermostat switch located with respect to the refrigerated plate to detect an excessive buildup in the thickness of an ice slab thereon, a second npn transistor including a base, collector and emitter, circuit means for connecting the base of said second transistor across the power source terminals including a biasing solenoid means, means for connecting said second solenoid means across the power terminals including the collector-emitter of said second transistor, a normally closed switch means in the energization circuit for the water pump circuit operated by said second solenoid means when the second transistor is conductive be tween the collector and emitter electrodes thereof, said second thermostat switch being responsive to an excessive buildup ofice thickness as produced when the grid switch is maintained closed during a refrigerated, recirculating water phase of operation to produce a biasing signal on the base of said second transistor to trigger said second transistor conductive thereby to energize said second solenoid means and condition said second switch means to decncrgize the water recirculation pump circuit thereby to terminate further buildup of ice thickness on the slab. 

1. A slab thickness limiting control circuit for an automatic ice maker of the type having an inclined refrigerated plate cooled by a refrigerant circuit and with water circulated thereover by an electric motor driven pump to build up a finished slab of ice thereon and further including an inclined ice cutting grid to receive the ice slab from the refrigerated plate when released therefrom by hot gas defrost means and with the grid serving to cut the slab into individual cubes comprising; first and second mercury ice thickness control thermostatic control switches, a normally opened grid switch being closed when a slab of ice is receiver on the cutting grid from the freezing plate, a first transistor having a base, an emitter and a collector, a second transistor having a base, an emitter and a collector, a DC power source including first and second terminals thereon, means including first and second biasing resistors connected respectively to the base of said first transistor and the base of said second transistor, normally closed first switch means for connecting the base of said first transistor to said first mercury ice thickness control thermostatic control switch, means including second switch means having a normally closed contact for energizing the refrigerant circuit to cool the plate and a normally open contact operable to a closed position for defining an electrical path for conditioning the hot gas defrost means to defrost the refrigerated plate during a slab release cycle of operation, said first mercury ice thickness control thermostatic switch being positioned a first preselected distance above the refrigerated plate to detect a buildup of a first predetermined thickness of ice in the slab and operative to open to produce a bias on the base of said first transistor when the grid switch is normally open to cause said transistor to be forwardly biased, means including a coil connected in circuit relationship with the emitter and collector of said first transistor and operable when said first transistor is conductive to open said normally closed first switch means and to shift said second switch means from its normally closed contact to its normally open contact, said second ice thickness control thermostatic switch being positioned a preselected greater distance above the refrigerated plate than said first ice thickness control thermostatic switch to detect an excessive slab thickness, means including said second ice thickness control thermostatic switch for producing a negative bias on the base of said transistor to cause said second transistor to be triggered conductive, a third normally closed switch means in series with said first switch means and being maintained normally closed to complete the water pump motor circuit, means responsive to said second transistor being triggered conductive to open said third switch means to deenergize said water pump when an excessive ice buildup is detected by said second ice thickness control thermostatic switch.
 2. A slaB limiting control circuit for association with a solid state refrigeration control circuit of the type including a first power transistor forwardly biased by means including an ice thickness sensor mercury thermostat to selectively position a solenoid operated relay to energize either a water circulating pump motor or a hot gas recirculation valve and wherein the base circuit of the transistor is connected to a switch that detects release of a slab of ice from an evaporator to direct a reverse bias signal to the transistor to cause the solenoid operated relay to be positioned in a water circulation mode when the slab of ice has been released from the evaporator plate the improvement comprising; circuit means including a second ice thickness sensor mercury thermostat switch for detecting an excessive buildup of ice slab thickness on the refrigerated plate, a second transistor having a base collector and emitter, circuit means including a biasing resistor in circuit relationship with said second ice thickness sensor mercury thermostat, normally closed switch means for completing the energization circuit for the water pump motor, circuit means connected across the collector emitter of said second transistor including means for operating said normally closed switch means to deenergize the water pump motor when said second transistor is turned on for current flow across its collector emitter electrodes, said second ice thickness sensor mercury thermostat being operative to trigger said second transistor to complete the conducting circuit between its collector and emitter to condition said normally closed switch means to terminate energization of said water pump motor thereby to prevent excessive build-up of ice slab thickness on the refrigerated plate when the slab switch is maintained closed during a refrigeration cycle of operation.
 3. A control circuit for preventing the excessive buildup of ice slab thickness on a refrigerated inclined plate over which water is circulated by an electric motor driven recirculating pump during a refrigeration phase and wherein an electrical defrost means is associated with the refrigerated plate to remove a slab of ice therefrom, the released slab of ice being received by an energized electrical grid for cutting the slab into a plurality of cubes of ice comprising: Circuit means defining an energization circuit across a power source for the pump motor including first and second switch means, first and second electrically energizable solenoid means for positioning said first and second switch means to control the energization of said pump motor circuit, second circuit means for energizing the defrosting means including a back contact selectively engageable by said first switch means when operated to deenergize the pump motor, means for selectively controlling energization of said first solenoid means including a first transistor having a base, electrode and emitter electrodes connected in circuit with said first solenoid means, means for directing a biasing current to the base of said first transistor including a normally closed third switch actuated by said first solenoid means and further including a first ice thickness sensing mercury thermostat, means including a tilt switch responsive to movement of a slab of ice onto the cutting grid having one terminal thereof connected to the base of said first transistor and the opposite terminal thereof connected to the power source and operative when a slab of ice has been released onto the grid to terminate the base biasing signal to said first transistor to interrupt the energization circuit for said first solenoid, and means for preventing an excessive buildup of ice on the refrigerated slab including a second ice thickness sensor mercury thermostat positioned to detect an excessive buildup of ice slab thickness, a second transistor having base, collector and emitter electrodes, said second solenoid means being connected across the collector emitter electrodes of said second transistor for enErgization across the power source, biasing means including said second ice thickness sensor mercury thermostat for directing a signal to the base of said second transistor when an excessive ice slab thickness is detected to condition said second transistor to energize said second solenoid means thereby to condition said second switch means to terminate energization of the water pump motor when said slab switch is maintained closed following a previous defrost cycle.
 4. A control circuit for preventing excessive buildup of thickness in a slab of ice on a refrigerated plate in an automatic ice maker of a type including a refrigerated plate having water recirculated thereacross by an electric motor driven pump to form an ice slab thereon and with an ice harvest cycle being initiated when a predetermined ice slab thickness occurs and including means for defrosting the refrigerated plate to cause the formed ice slab to slide onto an ice cube cutting grid, the control circuit comprising in combination: a first ice thickness control thermostat switch, a grid switch actuatable when a slab of ice slides from the refrigerated plate onto the cutting grid, a first npn transistor having a base, an emitter, and a collector, a direct current power source having first and second terminals, one of said terminals being connected to said grid switch and said first thermostat switch, means including a biasing resistor for connecting the other of said power source terminals to the base of said first transistor, normally closed first switch means connecting said transistor base through said first thermostat switch to the other power terminal, second switch means having a normally closed contact for energizing means for circulating water across the refrigerated plate and a normally open contact, means including said normally opened contact to define an energization circuit for operating the defrost means for the refrigerated plate, said biasing resistor being selected so that when a predetermined temperature is sensed by siad first ice thickness control thermostat switch a current flow will be directed through the base emitter circuit of said first transistor to cause it to be conductive between the collector-emitter circuit thereof, first solenoid means connected in circuit with the collector-emitter circuit of said first transistor and energizable when said first transistor is triggered conductive to condition said first switch means to open and to condition said second switch means to close the normally open contact thereof thereby to terminate water circulation and to initiate defrost and an ice slab harvest cycle, a normally open ice slab grid switch connected between the base of said first transistor and one of the power source terminals and operative when the slab of ice has been released to reverse bias the base emitter circuit of said first transistor to terminate energization of said first solenoid means thereby to terminate defrost of the refrigerated plate and to reinstitute recirculation of water across the refrigerated plate, and means for preventing excessive build-up of ice when the normally open slab sensing grid switch is closed by the presence of an uncut ice slab on the grid including a second ice thickness control thermostat switch located with respect to the refrigerated plate to detect an excessive buildup in the thickness of an ice slab thereon, a second npn transistor including a base, collector and emitter, circuit means for connecting the base of said second transistor across the power source terminals including a biasing solenoid means, means for connecting said second solenoid means across the power terminals including the collector-emitter of said second transistor, a normally closed switch means in the energization circuit for the water pump circuit operated by said second solenoid means when the second transistor is conductive between the collector and emitter electrodes thereof, said second thermostat switch being responsive to an excessive buildup of ice thickness as produced when the grid switch is maintained closed during a refrigerated, recirculating water phase of operation to produce a biasing signal on the base of said second transistor to trigger said second transistor conductive thereby to energize said second solenoid means and condition said second switch means to deenergize the water recirculation pump circuit thereby to terminate further buildup of ice thickness on the slab. 